This structure of the intramembrane portion (residues 1-125) of the
KcsA K+
channel from Streptomyces lividans was solved by J. H. Morais Cabral, R.
MacKinnon, & M. Zhou in 2001.

Suggested display options:

Use the right mouse button to color chain to distinguish the
4 copies of the protein.
Display as cartoon. Use the left mouse button to drag the image.

Note the tilt of the 2 transmembrane a-helices in each subunit.
The complex has been described as like a teepee or cone.

Select hetero-ligand
and display as ball and stick with color CPK.
In addition to K+ ions in the selectivity filter, a channel inhibitor will appear in the internal vestibule.
Try toidentify this compound.

Select
atom K and display spacefill to visualize more clearly the potassium ions.

The K+ ions are in the region called the selectivity filter, where the lumen exactly
corresponds to the diameter of K+. The selectivity filter consists of residues 75-79, the consensus sequence (T, V, G,
Y, G) found in all K+ channels.

Type into the command line select 75-79 and enter.
Use the mouse to change the
display to ball & stick, & color CPK.To zoom
in, left drag while holding down the shift key. Note the rings of oxygen atoms that define
the binding sites for K+.

Questions:
How many oxygen atoms of the selectivity filter interact with eachK+? Why are
cation-binding sites often rich in oxygen atoms?
Why
is it unlikely that there are actually 4 K+ ions within any
individual channel
at any time?

Follow the peptide chain in the region of
the selectivity filter. You can click on each amino acid to identify it. Question: Are most of the oxygen atoms that interact with K+
part of the peptide backbone or part of amino acid
side-chains (R-groups)?

Also view with the consensus sequence (select 75-79) displayed as
spacefill.
Note the close interaction of K+ with the oxygen
atoms.

The channel gate is where the transmembrane a-helices meet at the narrow end (the pointed end of the teepee).
Select protein-protein and change the
display to spacefill. Drag to view.Questions:
Is there any opening at the narrow end of
the channel?

Whatanionic
residues are located near each mouth of the channel?
How might you explain this common feature of cation-selective channels?